Clean up Decoder result handling

assets/parser.d.erb

@@ -36,23 +36,40 @@ class <%= @classname %>
 
     static class Decoder
     {
-        enum : uint
-        {
-            CODE_POINT_INVALID = 0xFFFF_FFFE,
-            CODE_POINT_EOF = 0xFFFF_FFFF,
-        }
-
-        struct DecodedCodePoint
+        struct Result
         {
+            enum : ubyte
+            {
+                SUCCESS,
+                EOF,
+                DECODE_ERROR,
+            }
+            ubyte result;
+            alias result this;
             uint code_point;
             uint code_point_length;
+
+            static Result success(uint code_point, uint code_point_length)
+            {
+                return Result(SUCCESS, code_point, code_point_length);
+            }
+
+            static Result eof()
+            {
+                return Result(EOF);
+            }
+
+            static Result decode_error()
+            {
+                return Result(DECODE_ERROR);
+            }
         }
 
-        static DecodedCodePoint decode_code_point(string input)
+        static Result decode_code_point(string input)
         {
             if (input.length == 0u)
             {
-                return DecodedCodePoint(CODE_POINT_EOF, 0u);
+                return Result.eof();
             }
             char c = input[0];
             uint code_point;
@@ -92,11 +109,11 @@ class <%= @classname %>
                 }
                 else
                 {
-                    return DecodedCodePoint(CODE_POINT_INVALID, 0u);
+                    return Result.decode_error();
                 }
                 if (input.length <= following_bytes)
                 {
-                    return DecodedCodePoint(CODE_POINT_INVALID, 0u);
+                    return Result.decode_error();
                 }
                 code_point_length = following_bytes + 1u;
                 for (size_t i = 0u; i < following_bytes; i++)
@@ -104,12 +121,12 @@ class <%= @classname %>
                     char b = input[i + 1u];
                     if ((b & 0xC0u) != 0x80u)
                     {
-                        return DecodedCodePoint(CODE_POINT_INVALID, 0u);
+                        return Result.decode_error();
                     }
                     code_point = (code_point << 6u) | (b & 0x3Fu);
                 }
             }
-            return DecodedCodePoint(code_point, code_point_length);
+            return Result.success(code_point, code_point_length);
         }
     }
 
@@ -238,13 +255,13 @@ class <%= @classname %>
             for (;;)
             {
                 auto decoded = Decoder.decode_code_point(m_input[(m_input_position + attempt_match_info.length)..(m_input.length)]);
-                if (decoded.code_point == Decoder.CODE_POINT_INVALID)
+                if (decoded == Decoder.Result.DECODE_ERROR)
                 {
                     lt.token = _TOKEN_DECODE_ERROR;
                     return lt;
                 }
                 bool lex_continue = false;
-                if (decoded.code_point != Decoder.CODE_POINT_EOF)
+                if (decoded != Decoder.Result.EOF)
                 {
                     uint dest = transition(current_state, decoded.code_point);
                     if (dest != cast(uint)-1)